add a libffi manifest

Summary: libffi is needed for getdeps python 3.8 update

Reviewed By: krallin

Differential Revision: D32459310

fbshipit-source-id: 6515eed088fb6de0091c2348b5732d1b635113e0

build/fbcode_builder/manifests/libffi

+[manifest]
+name = libffi
+
+[rpms]
+libffi-devel
+libffi
+
+[debs]
+libffi-dev
+
+[download]
+url = https://github.com/libffi/libffi/releases/download/v3.4.2/libffi-3.4.2.tar.gz
+sha256 = 540fb721619a6aba3bdeef7d940d8e9e0e6d2c193595bc243241b77ff9e93620
+
+[build]
+builder = autoconf
+subdir = libffi-3.4.2